People are generally aware of the most common causes of hallucinations, like schizophrenia and a really high fever. But lesser known are the more rare causes; here are six you probably haven't heard about.

Racism Is Unnatural, Study Says

There are two schools of thought about race. One school views race as a social construct, and that there is only one true race in humans: the human race. The other school disagrees with that idea, saying that the brain indeed reacts when it sees faces of other races, and holds that as proof that racism is innate.

Those that believe in the social construct idea argue that different ethnic groups were largely separated until the era of nautical travel, and that travel did not open up to the masses until airline travel, so there would be no evolutionary reason for the biological response in humans. And the two groups go back and forth, with neither side changing their mind.

Now, a study published in an upcoming issue of the Journal of Cognitive Neuroscience seems to suggest that racism is not innate, after all. The study, conducted by Eva H. Telzer at the University of California, Los Angeles and three colleagues, took 32 children of both European American and African American ancestry and conducted functional MRIs on them while showing them European American and African American faces. Previous studies on race that used fMRIs on adults had found that the amygdala, the part of the brain that registers emotion and especially the detection of threats, lit up when seeing faces of other races.

But in the study on children, who were between the ages of 4 and 16, the amygdala did not light up until the participants hit the age of 14. What's more, the amygdala did not light up equally for everyone. For children who had grown up with ethnically diverse peer groups, the amygdala's effect disappeared entirely. And, in children whose amygdala did respond to the pictures of African American faces, that effect existed, no matter what race the child was. That means that African American children viewed African American faces with fear, even though these faces were ostensibly not the "other".

The study authors say, "[These] findings suggest that neural biases to race are not innate and that race is a social construction, learned over time.''

Researchers are not clear on the exact direction of the correlation; do children with the weaker response in the amygdala seek out more diverse peer groups, or is it the other way around? Is puberty the reason that children view race less mildly? It does seem likely that children are more wary of other groups - but the meaning of enemy groups can and has changed substantially over time. Whatever the case, this study makes clear that this debate will continue for at least a little while longer.